1. Field of the Invention
The present invention relates to a broadband wireless communication system. More particularly, the present invention relates to an apparatus and method for determining an UpLink (UL) transmission format in a broadband wireless communication system.
2. Description of the Related Art
In the next generation communication system, also known as the 4th Generation (4G) communication system, research is being conducted to provide a Quality of Service (QoS) with a data transfer speed of about 100 Mbps. An example of a 4G communication system is an Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system. The IEEE 802.16 communication system employs an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme so that a broadband network can be supported by a physical channel of the wireless communication system.
In the IEEE 802.16 communication system, a Base Station (BS) obtains channel state information with respect to a Mobile Station (MS), and determines an UpLink (UL) transmission format of the MS. Examples of the UL transmission format include a Modulation and Coding Scheme (MCS), the number of slots, etc. The channel state information may be represented by UL Transmission (Tx) power. Power control for regulating the UL Tx power is classified into one of inner loop power control and outer loop power control. In the inner loop power control, Tx power of the MS is adjusted to a reference value of a preset Signal to Interference and Noise Ratio (SINR). The reference value is referred to as a set point. In the outer loop power control, the set point is changed according to a channel state of the MS.
A process of determining the UL transmission format of the MS by the BS will now be described below. First, the BS determines an MCS level for an MS so as to control interference. In this case, the BS allows a high MCS level to be assigned to an MS located near the BS, and allows a low MCS level to be assigned to an MS located near an edge of the BS's service area. Even when a plurality of MSs use the same MCS level, a different SINR may be required according to a channel state of each MS. Therefore, the BS performs the outer loop power control and thus determines a set point such that a Burst Error Rate (BER) of the MS can be satisfied. The set point is used to determine the Tx power of the MS. The Tx power of the MS is used to determine the number of slots allocated to the MS.
However, when the aforementioned method of determining the UL transmission format is used, system performance deteriorates in the following two environments.
In a first environment, channel quality rapidly deteriorates. As a result, a time point at which the channel state information is obtained and a time point at which the UL transmission format is determined by using the channel state information are different by several frames. In other words, the BS determines the UL transmission format by using the channel state information which was obtained several frames before the current frame. Thus, when the channel quality rapidly deteriorates, the MS continuously increases Tx power in order to satisfy a Reception (Rx) SINR of the BS. However, the amount that the Tx power can be increased is limited. Accordingly, at a certain time point, the maximum Tx power will be reached. In this case, since the UL Tx power is calculated for each sub-carrier, the BS cannot know whether the Rx SINR is satisfied or not. That is, even when the Tx power reaches its limitation and thus the MS is using a Tx power that is lower than what is needed, the BS may estimate that a low Tx power is being used because the channel quality is good. In this case, the BS determines a transmission format such that the MS can use numerous slots. As a result, the Rx SINR is continuously reduced, thereby decreasing system performance.
In a second environment, a required BER is not satisfied for a specific MCS level. For example, when multiple paths between the MS and the BS significantly vary in length, even if the MS is located near the BS, the required BER will not be satisfied for some of the higher MCS levels. The MCS level is determined so as to control interference and is based on a Tx power value. Therefore, the required BER may not be satisfied in a channel condition where the Rx SINR is restricted. That is, if a maximum Rx SINR is less than an SINR required for the MCS level, the required BER is not satisfied.
Therefore, when the BS determines the UL transmission format of the MS, system performance deteriorates in the aforementioned first and second environments. That is, a UL scheduling method and an outer loop control method which are conventionally used in the broadband wireless communication system cannot prevent deterioration of system performance in the aforementioned first and second environments.